1. Field of the Invention
Generally, this invention pertains to an optical wavelength add/drop multiplexing unit and more specifically to a tunable optical wavelength add/drop multiplexing unit for both digital and analog applications.
2. Description of the Related Prior Art
Last decade has seen an explosion in applications involving fiber optic technology. Fiber optic systems have primarily been used to develop very high information capacity transmission systems. The information capacity or xe2x80x98capacityxe2x80x99 of an optical communication link has doubled every year during the past half decade or so. The technology has reached a state of maturity where large systems manufacturers are routinely deploying wavelength division multiplexed (WDM) systems with total capacity exceeding 400 Gigabits/second on a single optical fiber. Such high capacity transmission systems are playing a critical role in revolutionizing today""s society, from e-commerce to e-mail and from voice-over-IP to plain old voice. The advent of WDM systems and the component used in WDM systems have also positive benefits in other areas including analog transmission systems like microwave photonic systems and fiber optic sensor systems, both of great importance in numerous applications for governmental as well as the commercial market.
Almost all of the WDM based optical communications systems deployed so far can be classified as optical transport systems or more commonly known as point-to-point communications systems. In such systems, relevant information (e.g., internet data traffic) is electronically multiplexed to a high bit rate. The multiplexed electrical information is then imposed upon an optical amplitude modulator and transported via optical fiber from point A to B (as shown in FIG. 1). If this process is repeated at several wavelengths then the optical transport system is referred to as a WDM system. This approach has become xe2x80x9cthe standardxe2x80x9d approach for high bit rate optical transport systems. The invention of optical amplifiers (e.g., erbium doped fiber amplifiers) have greatly enhanced the practicality and elegance of WDM systems and is primarily responsible for accelerating the rate of deployment of WDM systems and also the number of multi-millionaires in the country. Such point-to-point optical transport systems are considered xe2x80x98presentxe2x80x99 generation systems and are commercially available from various well known corporations like Lucent Technologies, Nortel Networks, Cisco Systems and Ciena.
The next generation of optical communications systems are expected to provide significantly greater functionality than just transport of optical signals from point A to point B. Optical transport systems are expected to evolve into true optical networks. Optical networks will allow routing and reconfiguration of traffic in the optical domain. In present generation optical transport systems routing of data is done primarily in the electrical domain. In other words, the optical signals are converted from optical to electrical domain at their destination and are then electrically manipulated such that certain traffic is dropped while new traffic is added. That is the main reason why high speed electrical routers are in great demand. In future optical communication systems the manipulation and routing of optical channels is expected to occur in the optical domain. Naturally, at some point within the network, optical signals will have to be converted into electrical signals but that conversion will take place towards the xe2x80x98edgexe2x80x99 of a given network rather than at the xe2x80x98corexe2x80x99 of the network. This is expected to greatly enhance network speed, allow for dynamic reconfigurability, and enhanced reliability. The cost per bit is also expected to drop due to the deployment of an optical layer. In order to achieve a true all-optical layer within a data network, devices which allow adding and dropping data channels in the optical domain will be essential. In order to facilitate dropping and adding wavelengths in WDM optical networks, there will be tremendous demand for wavelength agile products. Wavelength agile products are products which allow you to manipulate optical signals without converting them into the electrical domain, Such products are sometimes also referred to as wavelength management products.
One of the products that will be in high demand will be a tunable wavelength add/drop multiplexers (t-WADM). Fixed wavelength add/drop multiplexers are already becoming commercial but these products require that the wavelengths to be dropped at a specific site (commonly known as a node) be known ahead of time. Fixed notch filter, typically made from fiber Bragg gratings, are utilized to make such fixed wavelength add/drop multiplexers. However, optical networks of the future require that a node be established within the network where any or all wavelengths can be dropped on demand. In other words, the number of wavelengths to be dropped or added in an optical network will be a dynamic parameter which will depend on local traffic demands and changing customer needs. Programmable or tunable all-optical wavelength add/drop devices will become crucial in such networks. In short, the need for dynamic reconfigurablity at the optical level in a WDM transport network will require programmable or tunable wavelength add/drop multiplexers (t-WADM) at numerous locations in the network.
The t-WADM also can be used in applications not involving optical communication. For instance, optical fiber systems are seeing a lot of use in defense related microwave photonics applications. Optically controlled phase array radars are now being deployed on a trial basdis in the Navy. Such optically controlled phased array radars also exploit the WDM technology primarily-developed for optical communication systems but extremely beneficial in more defense related applications. The t-WADM products can also be used in high count fiber sensor systems typically used for advanced underwater applications like acoustic arrays, Fiber optic acoustic arrays are now scheduled for fleet insertion. Acoustic arrays are also being used for developing advanced underwater acoustic surveillance arrays. These arrays already utilize WDM architecture and the t-WADM can potentially be utilized in fiber sensor systems.
The object of this invention is to provide an optical system which allows dropping and adding of optical data channels in wavelength agile optical communications networks.
This and other objectives of this invention are achieved by a tunable wavelength add/drop (t-WADM) device utilizing an integrated optic digital switch (DOS) which is an integrated optic on/off switch with high extinction ratio and a digital response curve. The t-WADM is comprised of a multiwavelength input (which serves as a data input port), a low loss optical circulator or an optical coupler, a wavelength division de-multiplexer which splits the input multi-wavelength data stream into its individual components, a modified multi-channel DOS, a telecommunications grade optical fiber, and a wavelength multiplexer for adding optical data channels. The input multi-wavelength data stream from a network is sent to a wavelength demultiplexer where it is demultiplexed into individual wavelengths which are applied to an array of Y-branch digital optical switching devices controlled by a computer. If a specific wavelength is to be dropped, it is diverted towards a branch of a given switch which has a fiber pig tail attached. If a specific wavelength is to be sent through (neither dropped or added) then the signal is diverted towards a mirrored end of the Y-branch, where it is reflected back towards the wavelength de-multiplexers to the circulator and goes out the output end of the t-WADM. If a wavelength xe2x80x98slotxe2x80x99 has ben vacated by dropping a channel then a new data stream may be added in that slot.